In systems of the present type, it is usually critical that the moving members be able to repeat their movements exactly and instantaneously. An example is the use of such systems for the control of the opening and closing events of engine valves or the actuation of a fuel injector. Commonly, in such systems, an electroexpansive module, such as a piezoelectric motor, functions to drive a relatively large diameter piston which is in hydraulic fluid communication with a smaller diameter piston. In this way, the downward displacement of the larger piston resulting from actuation of the piezoelectric motor may be small while the downward displacement of the smaller piston will be considerably greater because the downward displacement of the smaller piston is equal to the downward displacement of the larger piston multiplied by a quotient obtained by dividing the effective surface area of the larger piston by that of the smaller piston. In such systems, the space or chamber between the larger piston and the smaller piston and the hydraulic fluid which fills the chamber, deserve important consideration.
In prior art systems, as shown in U.S. Pat. No. 3,501,099, entitled "Electromechanical Actuator Having an Active Element of Electroexpansive Material", which patent issued to Benson on Mar. 17, 1970, the space was closed, meaning the fluid in the space could not be resupplied in the event of a leak, or bled, in the event of a surplus. Disadvantages of such systems were that because there were moving parts, leakage out of the space was not uncommon, and also, after repeated heating and use of the hydraulic fluid, its viscosity and other characteristics could change. Obviously, such events could change the reaction time and movement of the smaller piston resulting in timing problems of other events such as fuel injection, air intake, and exhaust outlet.
A later prior art system, disclosed in British patent specification No. 1,569,638, entitled "Internal Combustion Engine", published on Jun. 18, 1980, was intended to solve the leakage problem by providing a resupply line having a ball check valve therein to the fluid chamber. However, there was no ability to bleed excess fluid.
Apparently, no one has seen a need to provide a bleed line to the fluid chamber, apparently figuring that because the volume of the chamber stays constant, there should be no need to bleed the chamber. However, this is not necessarily the case. After the piezoelectric motor has been energized and then shut off, the piezoelectric motor may retain some thermal expansion. Or, there may be thermal expansion of the hydraulic fluid in the fluid chamber as it heats up, thus preventing the small piston from returning fully to its first position. Thus, the large piston is unable to return fully to its first position and the biasing force against the smaller piston trying to return it to its first position is not great enough to overcome the pressure retained in the fluid chamber, thus the smaller piston will not return fully to its first position.
The provision of a bleed line or a pressure equalization line to the fluid chamber will allow the pressure in the fluid chamber to be dissipated, thus allowing the biasing force of the smaller piston to overcome the hydraulic pressure and return fully to its first position, in spite of the thermal expansion of the piezoelectric motor or hydraulic fluid, and this is an object of the present invention.